This is the name of a Starcraft 2 show by Day9. On mondays he asks users to submit replays where the players are constrained by particular rules to make the games more interesting. Funday Monday typically gains larger audiences than Day9's other shows.

The most popular segment of the Day9 Dailies, the StarCraft II video podcast of Sean "Day9" Plott.

The basic idea: Day9 sez, "Here's an arbitrary restriction(s) that usually negatively affects your gameplay. Send me a video of a match in which you adhere to said restriction (singular or plural), and maaaaaaaybe I'll feature it on my vodcast."

The results:
-Ladder rankings for many people get fucked over. Some think this is the intent of Day9.
-Many people hope their matches will be featured on Day9's weekly Funday Monday segment.
-Said people usually use Day9 as an excuse for their bad gameplay when attempting Funday Mondays.

*at beginning of match*
Player 1: glhf
Player 2: u2
*at end of match, when Player 2 is losing rather badly*
Player 1: You've been playing rather badly. Kinda odd...
Player 2: Day9 made me do it!
Player 1: oic